A need exists for improved influenza vaccines. Current vaccine strategies against influenza focus on generating robust antibody (humoral) responses against hemagglutinins. Because of the high degree of antigenic drift among circulating influenza strains over the course of a year, vaccine strains must be reformulated specifically for each influenza season. Although annual (or seasonal) influenza vaccines are successful to varying degrees in different age categories, more effective protection is clearly needed particularly for the young and the elderly. Furthermore, there is a major, permanent risk that reassortant viruses will evolve which have acquired very different HA (hemagglutinin) genes in a process known as “antigenic shift”. This would create a public health emergency, as current influenza vaccines rely essentially on the HA antigen.
Influenza is an enveloped, single-stranded, negative-sense RNA virus in the Orthomyxoviridae family of viruses, divided into 3 major types: A, B, and C. Influenza A viruses infect a wide variety of animals, including humans, birds, pigs, horses, bats and many others, although the tropism of any particular influenza virus is generally highly adapted to a particular host. Influenza B viruses infect a smaller number of species, namely humans and seals, but are still a substantial cause of annual influenza epidemics. Most human influenza infections are caused by influenza A or B; influenza C viruses, which infect humans and pigs, rarely account for serious human infections or epidemics (Lamb).
The current inactivated influenza virus vaccines induce antibodies that protect against closely related virus strains. Currently licensed vaccines mainly induce strain--specific neutralizing antibodies against hemagglutinin (HA), the main antigenic determinant on the surface of the virus, which is highly immunogenic, and can prevent disease caused by infection with a matching virus strain. However, HA has substantial antigenic variation which excludes its use alone in a vaccine designed to provide broad protection. For this reason, alternative vaccine strategies that generate protective responses directed against less variable targets are of great interest.
Natural infection with influenza A virus induces both humoral and cellular immunity. Long-lasting cellular immunity is directed predominantly against conserved, internal viral proteins, such as the nucleoprotein (NP). NP antigen is immunogenic in humans following natural infections, but the cytotoxic T lymphocytes that are induced have a short life-span (McMichael a, McMichael b).
Cellular immunity against NP is valuable, as it is directed against different variants of NP epitopes, and NP-targeting DNA vaccines have induced cross-protective immunity in animals (Schotsaert).
The nucleoprotein (NP) antigen has long been recognized as a highly conserved antigen: even the most divergent influenza A strains share 90% identity in the NP proteins they encode (Gorman, Xu). Antigenic changes to NP are rare and only occur to a minor extent (Staneková).